Closed-cell spray foam and rigid foam board have similar R-values per inch, but they behave differently on a real building: spray foam conforms to every gap and self-seals, while board comes in flat sheets that leave seams and edges to tape. For irregular or hard-to-reach assemblies, that difference decides it.
The trade-offs
- Rigid board: predictable, factory-consistent, good for flat continuous surfaces (exterior sheathing, flat roofs). But every seam and penetration is a hand-sealed joint that can fail.
- Spray foam: monolithic and gap-filling - no seams, self-flashes around pipes and odd shapes, adheres directly. Costlier and needs a pro.
Which where
Flat, open, accessible surface on a budget -> board can win. Irregular cavities, rim joists, curved metal, tight retrofits -> spray foam, because board simply can't seal them. Compare closed vs open cell or get a quote.